Our research focuses on the regulation of gene expression, in particular the mechanisms controlling cellular and viral mRNA expression. A critical step in the mRNA metabolism is the transport of the mRNA from the nucleus to the cytoplasm. Analysis of retroviral systems, pioneered by research on HIV-1, have shed light into some important aspects of nuclear mRNA export and these studies have provided critical insight into mechanisms governing cellular mRNA and protein transport. We are utilizing retroviral systems to identify and study mechanisms of mRNA metabolism using a combination of biochemistry, functional genomics, and proteomics. The dissection of the mechanisms of posttranscriptional control and nucleocytoplasmic trafficking of macromolecules are relevant to understand processes involved in cellular gene expression as well as virus expression. Since posttranscriptional regulation is an essential step in virus propagation, our studies also focus on the understanding of this regulatory step in pathogenesis of HIV and SIV. Recent data from my lab identified the cellular PSF protein as a factor interacting with distinct HIV-1 mRNAs and inhibiting their expression, hence virus production. The role of PSF in the posttranscriptional regulation of cellular mRNAs is under investigation. The identification of CTE and CTE-related RNA transport elements as well as their receptor, the cellular TAP/NXF1 protein, enabled us to further dissect its mechanism of function. NXF1 is also the key receptor for cellular mRNAs, and we demonstrated that this function is conserved in metazoa. We also found that NXF1 binds to the splicing factor U2AF35 which provides a novel link between splicing and export of mRNA. We are currently examining the role of the other members of the NXF family. In collaboration with George Pavlakis (Vaccine Branch), we study a novel RNA transport element RTE. The mechanism of function of RTE is still under investigation but it appears that RTE has hijacked the NXF transport pathway. Interestingly, we found that a combination of RTE and CTE synergistically increases production of poorly expressed viral mRNAs. Since the function of these elements is conserved in mammalian cells, the use of the RTE-CTE combination provides a simple method to improve gene expression to levels otherwise only achieved via more cumbersome RNA optimization.